1. Field of the Invention
The present invention relates to an imaging device.
Priority is claimed on Japanese Patent Application No. 2008-333578, filed Dec. 26, 2008, the content of which is incorporated herein by reference.
2. Description of Related Art
When a subject is picked up using an imaging element, such as a CCD (Charge Coupled Device) or a CMOS (Complementary Metal Oxide Semiconductor) image sensor, a technique known as prolonged exposure which obtains a proper amount of exposure by lengthening the exposure time if the subject is dark has been used.
In the imaging by this prolonged exposure, an imaging device which performs exposure multiple times in a predetermined short exposure time and adds image signals obtained in each exposure, thereby obtaining an image of prolonged exposure in which an S/N (Signal-to-Noise ratio), which represents the amount of noise to a signal, is high is disclosed (refer to Japanese Unexamined Patent Application, First Publication No. 2003-69897).
Additionally, the above publication discloses displaying an image during the prolonged exposure, which is obtained by adding respective image signals obtained in a short exposure time while performing the prolonged exposure, on a display device provided in the imaging device, thereby confirming whether or not the image obtained during the prolonged exposure (the image data which will finally be recorded) is an image that the user of the imaging device wants to obtain.
FIG. 6 is a block diagram showing a schematic configuration of a related imaging device disclosed in the above publication.
In FIG. 6, the imaging device 500 includes a lens group 511, a solid-state imaging element 512, an imaging circuit 513, an A/D converter 514, a lenses driving circuit 515, a detachable memory 520, an image processing circuit 521, an compression/expansion unit 522, a built-in memory 525, an LCD (Liquid Crystal Display) 530 for image display, an LCD driver 531, a system controller 540, a nonvolatile memory 541, and an operation unit 546.
When an image is taken by the imaging device 500, first, an optical image of a subject which has passed through the lens group 511 is formed on the solid-state imaging elements 512, such as a CCD or CMOS image sensor. After that, the optical image formed by the solid-state imaging elements 512 is converted into electrical signals (hereinafter referred to as “pixel signals”) according to the quantity of incident light and the converted signals are output to the imaging circuit 513.
Subsequently, the imaging circuit 513 converts the pixel signals input from the solid-state imaging element 512 into analog image signals of the subject, and outputs the converted signals to the A/D (Analog to Digital) converter 514 at the subsequent stage.
Subsequently, the A/D converter 514 converts the analog image signals into digital image signals (hereinafter referred to as “image information”) in an analog-to-digital manner, and temporarily stores the converted signals in the built-in memory 525. For example, an SDRAM (Synchronous Dynamic Random Access Memory), or the like is used for the built-in memory 525, and is a high-speed nonvolatile temporary storage memory utilized also by the image processing circuit 521 which will be described later.
The image processing circuit 521 creates image data by performing conversion processing of color information, conversion processing of the number of pixels, or the like on the image information temporarily stored in the built-in memory 525, and outputs this created image data to the compression/expansion unit 522.
The compression/expansion unit 522 creates compressed image data by compressing the image data processed by the image processing circuit 521 by using, for example, a JPEG (Joint Photographic Experts Group) compression technique, or the like, and records this compressed image data on the detachable memory 520, such as a memory card.
Additionally, when an image picked up by the imaging device 500 is displayed, image processing required for display is performed on the image data processed by the image processing circuit 521, and then, the image data is displayed on the LCD 530 for image display via the LCD driver 531. Additionally, when the image information recorded on the detachable memory 520 is displayed on the LCD 530 for image display, first, the compressed image data stored in the detachable memory 520 is read and is expanded by the compression/expansion unit 522, that is, the same image data as the image data processed by the image processing circuit 521 is created. Thereafter, the image data is subjected to the image processing required for display by the image processing circuit 521, and is then displayed on the LCD 530 for image display via the LCD driver 531 similarly to a case where the image picked up by the imaging device 500 is displayed.
The system controller 540 controls the operation of the imaging device 500 according to input from a release button or arrow key (not shown) which is operated by a user of the imaging device 500. Additionally, the system controller 540 controls the driving of the lens group 511 via a lenses driving circuit 515.
In the above-described configuration in the above publication, the respective image information obtained by multiple exposures is temporarily stored in the built-in memory 525, the respective image information temporarily stored in the built-in memory 525 is added, and the added image information is displayed on the LCD 530 for image display.
Additionally, in the above publication, two methods are disclosed as the method of adding the image information obtained by multiple exposures, and obtaining an image of prolonged exposure with high S/N. FIGS. 7A and 7B are block diagrams showing a schematic configuration which adds image information in the related imaging device.
The first method, as shown in FIG. 7A, is a method which includes a plurality of built-in memories 525, and which, when each piece of image information is temporarily stored in each built-in memory 525 and displayed on the LCD 530 for image display, reads the image information from each built-in memory 525 to add the image information by an adder 526.
The second method, as shown in FIG. 7B, is a method which includes one built-in memory 525 and which adds image information temporarily stored up to the last time by an adder 526 before the image information is temporarily stored, and repeats the operation of temporarily storing the added image information as new image information.
FIG. 8 is a drawing showing the relationship among the exposure timing, the signal level and noise level of an obtained image, and the display, in a prolonged exposure operation of the imaging device of the above publication. An example where, when imaging by prolonged exposure is performed, exposure is performed four times in a shorter exposure time than the time of prolonged exposure, and the obtained four images are added is shown in FIG. 8.
In FIG. 8, when a shutter is brought into an “open” state, first, exposure 1 is started and imaging of a subject is performed. If the predetermined exposure time has passed, once the exposure is completed, the image information 1 obtained by the exposure 1 is read. The read image information 1 is subjected to image processing, and image data 1 is created.
Subsequently, the reading of the image information 1, while next exposure 2 is started simultaneously, and imaging of the subject is continued. If the predetermined exposure time has passed, the exposure is completed, image data 2, which is subjected to image processing similarly to the operation of the exposure 1 and is added to the image data 1 created by the operation of the previous exposure 1, is created. In the created image data 2, the signal level of the image data is doubled.
Similarly, as image data 3 and image data 4 are added, the signal level of image data is increased to a multiple of the number of image data added. This makes it possible to obtain a proper amount of exposure (the signal level of image data) even when the subject is dark.
Additionally, in the imaging device of the above publication, the image data picked up during prolonged exposure, i.e., during a period in a state where the shutter is in an “open” state, is displayed by performing the image processing required for display after the image data is created. In the example of FIG. 8, the image data 1 to the image data 3 before the image data 4 that is the final image data obtained by adding four kinds of image data are displayed.
That is, it is possible to display image data of the number of times of display during prolonged exposure, which is obtained by subtracting 1 from the number of image data.